To quickly acquire necessary data in a database, it is important to speed up searching the database. Moreover, for instance, for analysis using big data that exists somewhere in the Internet or the like, capability of searching a very high volume of data at a high speed is required.
With regard to speeding up a text search in terms of such search, Patent Literature 1 discloses a technique in which indexes (descriptor tables) on the appearance positions of characters constituting each of all trigrams, such as “abc” and “bcd”, are created from a text to search and, if a search word of “abcd” is specified, the appearance positions of characters in an “abc” index and the appearance positions of characters in a “bcd” index are checked to search for contiguous positions (paragraphs 0002 to 0009, FIGS. 2 and 4). And now, such a technique is known as an n-gram method (a trigram method, if an n-gram consists of three characters).